Acoustic signal translating device having a propagating medium composed of a plurality of effectively distinct signal translating paths of mutually different effective lengths



Aug. 19, 1969 v 3,462,714

, ACOUSTIC SIGNAL TRANSLATING DEVICE HAVING A PROPAGATING I R, ADLER .MEDIUM COMPOSED OF A PLURALITY 0F EFFECTIVELY DISTINCT SIGNAL TRANSLATING PATHS. OF

MUTUALLY DIFFERENT EFFECTIVE LENGTHS ffdl Loud Inventor Robert Adler By 295. Afforneyi United States Patent ACOUSTIC SIGNAL TRANSLATING DEVICE HAV- ING A PROPAGATING MEDIUM COMPOSED OF A PLURALITY 0F EFFECTIVELY DISTINCT SIG- NAL TRANSLATING PATHS 0F MUTUALLY DIF- FERENT EFFECTIVE LENGTHS Robert Adler, Northfield, 111., assignor to Zenith Radio Corporation, Chicago, Ill., a corporation of Delaware Filed Oct. 3, 1966, Ser. No. 583,634 Int. Cl. H03h 9/00, 7/30 U.S. Cl. 333-72 6 Claims ABSTRACT OF THE DISCLOSURE An acoustic wave propagating medium is composed of a plurality of acutally or effectively distinct signal translating paths of mutually diiferent effective lengths. An input transducer responds to an input signal to create acoustic waves in the medium, and an output transducer coupled to the medium responds to the waves to develop an output signal. By correlating the relationship between the different lengths, the selectivity of the device is enhanced so that it serves advantageously as a selective filter. In one embodiment, physically separate acoustic delay lines of different lengths are provided between the input and output transducers, preferably with different cross-sectional areasf In another embodiment, a liquid such as water is used as the acoustic wave propagating medium, and an acoustic phase grating is included at an angle to the path of acoustic wave propagation, which angle may be adjustable to provide for selective tuning.

This invention pertains to solid-state acoustic time domain filters. While the filters disclosed are theoretically capable of selectivity at all frequencies, practical considerations make it particularly advantageous for use in the very high frequency range (VHF) and the filters are, therefore, described in that environment.

It is known that waves caused to propagate along a medium acoustically traverse the medium in finite time interval. Hence, this mechanism has found use in delay lines and it is recognized that such delay lines may exhibit inherent filtering characteristics which impart a moderate degree of frequency selectivity to the device. It is a general object of the present invention to provide an acoustic filter which is capable of a much higher degree of frequency selectivity.

In the field of integrated circuitry, it has been extremely difficult to produce correspondingly miniaturized selective devices. Accordingly, it is one specific object of the present invention to provide an acoustic signal translating device capable of a high degree of selectivity and sufficiently miniaturized to be useful in integrated circuitry applications.

It is another object of the present invention to provide a new and improved solid'state acoustic signal translating device with a variable pass band.

A signal translating device constructed in accordance with the present invention comprises an acoustic wave propagating medium composed of a plurality of effectively distinct signal translating paths of different effective lengths. Means responsive to an input signal create acous' tic waves in the medium, and means coupled to the latter respond to the waves and develop an output signal.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements and in which:

FIGURE 1 is a partly schematic side elevational view of one embodiment of an acoustic filter;

FIGURE 2 is a similar view of another embodiment of such a filter; and

FIGURE 3 is a partly schematic side elevational view of yet another acoustic filter embodiment.

In FIGURE 1, a signal source 10 is coupled across a primary winding 11 of a transformer 13. The secondary winding 12 of transformer 13 is in turn coupled across piezoelectric transducer 15 by means of a highly conductive electrode film 14, such as gold, on one surface of transducer 15 and a highly conductive bonding layer 16, such as indium, on the opposed surface. A signal translating medium 17 is bonded to transducer 15 by layer 16 at its input end and is also bonded at its output end by virtue of another indium layer 18 to a piezoelectric output transducer 19. An inductor 21 is coupled to transducer 19 by connecting it across layer 18 and a gold layer 20 on the opposed surface of transducer 19. Inductor 21 is coupled to an inductor 22, forming transformer 23, which is in turn coupled across a load 24. Transformers 13 and 22 provide impedance matching appropriate to source 10 and load 24.

Signal translating medium 17 is made up of an array of delay lines 17a-17d in parallel and of various lengths. Each of the lines exhibits the property of propagating acoustic waves, and they may, for example, be constructed of a material such as quartz. The material need not be piezoelectric, although when it is advantage may be taken of that property by so affixing input and output electrodes as to combine or integrate the functions of transducers 15 and 19 into the lines themselves.

In operation, signal source 10 induces a signal across transformer 13 which is imparted to piezoelectric transducer 15. Transducer 15 converts the electrical signal into mechanical wave vibrations which are imparted to lines 17a-17d. Each of the delay lines propagates acoustic waves representing the input signal from its input end to its output end where the signal energy is imparted to transducer 19. Transducer 19 converts the wave energy into an electrical signal which is fed to load 24.

The aggregate output signal from the plurality of parallel acoustic delay lines, each of which is some time, AT, different in acoustic delay from each of the other lines, is a maximum for an input signal 'whose period is very close to AT or an integral fraction thereof. An input signal of any other period arrives with different respective phase at the output of each line, producing a lesser aggregate signal; an input signal whose period is sharply different from AT or an integral fraction of AT produces a weak output, while an input signal whose period is nearly AT (or an integral fraction thereof) produces nearly equally phased driving forces at the ends of the lines and thus delivers a strong output.

Thus, for a desired signal frequency having a corresponding period, each delay line differs in length from the next by an amount equal to one wavelength, or an integral multiple thereof. At this desired frequency, f the signal received by output transducer 19 is a maximum, whereas at any other frequency the signal is attenuated.

The degree of selectivity depends on the total number of such delay lines; when all lines transmit equal amplitudes, the first signal null in the conventional selectivity curve occurs at that frequency for which the delay between the first and the last line is something less than one period. The first frequency null f is given by the formula:

where n is the number of delay lines.

Because the output transducer response is the vector sum of a number of individual line responses, which vary with frequency only by adding delay in linear proportion to frequency change, the device is governed by equations analogous to those for the directional characteristic of an optical intensity grating, as contrasted with a phase grating.

The typical side lobes which appear in the frequency response characteristic of an intensity grating may be avoided in the present apparatus by tapering the amplitude response as between the different lines. One embodiment of the present apparatus incorporating such tapering is depicted in FIGURE 2, where the apparatus is like that of FIGURE 1 except for the delay lines themselves. In this instance, the inner delay lines have a larger crosssectional area than the others and thus carry more power than the others. Specifically, the output ends of the lines are located in a plane and the outermost delay lines 25a, 25c have a relatively small cross-sectional area in comparison with the innermost delay line 25c; intermediate lines 25b, 25d have an intermediate cross-sectional area.

In general operation, the apparatus of FIGURE 2 is identical with that of the apparatus of FIGURE 1. An electric signal produced by the source imparts an acoustic signal to the delay lines by way of transducer 15. This signal, after acoustic translation, is imparted to output acoustic transducer 19 which produces an electrical signal across load 24. However, as a result of the varied line areas, the frequency response characteristic is different in that the amplitude of the side lobes normally associated with the apparatus of FIGURE 1 is reduced in the apparatus of FIGURE 2.

As an example of practical dimensions for the apparatus of FIGURE 2, a time delay filter for 45 megahertz with about 10 percent bandwidth will now be described. Each individual delay line is of a metal having a sound velocity equal to 4.5 km./sec. The length difference between any two adjacent lines is one wavelength or 0.1 millimeter (.004 inch). Considering only the characteristics of the FIGURE 1 device, ten lines are needed to obtain a ten percent bandwidth. However, incorporating the tapered response feature of FIGURE 2 results in the use of twenty individual lines. The difference in length between the first and last line is thus 2 mm.

The driving transducer is, of course, wide enough to accommodate all twenty lines, and the spacing between adjacent lines is not critical, subject primarily only to manufacturing tolerances. Each line must be several wavelengths thick to avoid dispersion effects. In this example, a five wavelength minimum line thickness (.5 mm) is adequate and mechanically feasible.

It may be noted that such dimensions become larger as the frequency is lowered. For example, at 450 kilohertz, the wavelength is one centimeter. For a two percent bandwidth at this frequency, the line lengths would be between 50 centimeters and one meter. Thus, for much lower frequnecy operation a modified transversal acoustic filter is often to be preferred. Such a filter is composed of a single acoustical delay line, tapped at a number of points, and the output from all of the taps is combined.

To allow tuning of the filter to different frequencies, it is contemplated to provide means for varying the effective length of the individual delay line components. An embodiment incorporating this feature is depicted in FIG- URE 3 wherein the apparatus again is like that in FIG- URE 1 except for the construction of the delay elements as such. In this case, transducer 15 is coupled to a unitary signal translating medium 26 such as water. Within medium 26, an apertured plate 27 is disposed at an angle a to the direction of acoustic wave propagation in the medium. Medium 26 is coupled at the output end of the structure to transducer 19 which is disposed at the same angle on to the direction of acoustic wave propagation.

In operation, the filter of FIGURE 3 is similar to the apparatus depicted in FIGURE 1. Specifically, the signal produced by source 10 is converted to an acoustic wave which travels through signal translating medium 26 and is then imparted as an electrical signal across load 24 by way of transducer 19. In this embodiment, however, the acoustic wave travels as a bulk wave throughout the entirety of unitary medium 26 to plate 27, rather than as a series of finite waves each in one of a plurality of parallel delay lines.

Near the output end of medium 26, the acoustic wave impinges upon apertured plate 27. The openings in the plate effectively divide the bulk acoustic wave into a series of waves equivalent to those in a series of individual acoustic signal translating lines. Angling apertured plate 27 and transducer 19 is equivalent to varying the length of the effective lines and thereby forms a series of acoustic delay lines of varied length. This difference in length is controlled, in one embodiment, by selection of the spacing of the openings in the plate for a given angle at. The size of the openings in the plate may be varied across its width analogously to the change in cross-sectional areas of the delay lines in FIGURE 2. Externally selectable tuning is achieved by mechanically changing the angle a that the plate and transducer make with the direction of acoustic wave propagation; the effective difference in length between the successive delay lines thus is continuously variable.

Apertured plate 27 is an amplitude grating; at least one-half the incident power ordinarily is rejected. However, the plate may be made of material such as a light plastic which produces little reflection but does change the velocity of wave propagation enough to obtain about phase shift through the thickness of the plate between those wave components traveling in the plastic and those traveling in the water. The result is a phase grating which has better efficiency of power transfer at the point of maximum response.

At 45 megahertz, when water is used as medium 26, the wavelength is only 5 mm. The total path difference between the first and last effective lines corresponds to a 20-wavelength difference is only .67 mm.

Delay of an electrical signal by the use of acoustic waves is a concept unlimited by the particular mode of acoustic translation. Therefore, although the apparatus as described herein takes advantage of the properties of longitudinal acoustic waves, it is worth observing that similar principles may be used in conjunction with surface and shear acoustic wave modes.

It is evident that the present invention affords a new and improved filter of the acoustic variety which has substantial advantages over predecessor devices. Acoustic elements take the place of more cumbersome components normally associated with tuned circuitry and modifications allow tuning of the apparatus.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects. Accordingly, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. An acoustic signal translating device comprising:

an acoustic wave propagating medium composed of a plurality of effectively distinct parallel signal translating paths of mutually different effective lengths;

input transducer means responsive to input signals for impressing acoustic waves on said medium;

output transducer means coupled to said medium and responsive to said waves for developing an output signal;

and said medium being composed of a plurality of individual delay lines each coupled at one end to said input transducer means and at the other end to said output transducer means, in which said delay lines individually carry unequal amounts of power.

2. A device as defined in claim 1 in which said delay lines individually have unequal cross-sectional areas.

3. A device as defined in claim 2 in which the output ends of said delay lines are aligned generally in a' plane and the central one of said lines has a cross-sectional area larger than that of the lines to either side thereof.

4. An acoustic signal translating device comprising:

an acoustic wave propagating medium composed of a plurality of effectively distinct parallel signal translating paths of mutually different effective lengths;

input transducer means responsive to input signals for impressing acoustic waves on said medium;

and output means coupled to said medium and responsive to said waves for developing an output signal, wherein the acoustic lengths of said lines differ by nAT, wherein n is an integer and AT is the period of a signal desired to be selected by the device.

5. An acoustic signal translating device comprising:

an acoustic wave propagating medium composed of a plurality of effectively distinct parallel signal translating paths of mutually different effective lengths;

input transducer means responsive to input signals for impressing acoustic waves on said medium;

and output transducer means coupled to said medium and responsive to said waves for developing an output signal, in which said medium is a unitary substance, an apertured plate is disposed in said medium, and said output transducer means and said plate are disposed across said medium in a plane defining a selectably adjustable acute angle to the propagation direction of said waves.

6. An acoustic signal translating device comprising:

an acoustic wave propagating medium composed of a plurality of effectively distinct parallel signal translating paths of mutually different effective lengths;

input transducer means responsive to input signals for impressing acoustic waves on said medium;

and output transducer means coupled to said medium and responsive to said waves for developing an output signal, in which said medium is a unitary substance, an apertured plate is disposed in said medium, and said output transducer means and said plate are disposed across said medium in a plane defining an acute angle to the propagation direction of said waves, in which said plate is of a material exhibiting minimal reflection of said waves in said medium and which achieves a phase shift between the parts of said waves which travel through said material and the parts of said waves which pass through said apertures.

References Cited UNITED STATES PATENTS 1/1950 Olson. 6/1968 Parker 33330 US. Cl. X.R. 

